Eugene Kirpichov wrote:
> I took a toy problem - find the first node satisfying a predicate in a
> binary tree, started with a naive Maybe-based implementation - and
> experimented with 3 ways of changing the program:
> - Church-encode the Maybe
> - Convert the program into CPS
> - Defunctionalize the Church-encoded or CPS-transformed program
>http://hpaste.org/fastcgi/hpaste.fcgi/view?id=10686>> The link points to code, a benchmark and conclusion.
>> Conclusion:
> - Haskell implements Maybe well enough that it is not possible to do better
> - Defunctionalization and consequent optimization yields same
> performance as the one with Maybe
> - Non-simplified CPS and Church-encoded representations do bad
I'm not sure your example program is enough to justify the conclusions.
The main advantage of the Church-encoding of Maybe
type Maybe' a = forall b . b -> (a -> b) -> b
is that the latter has the pattern match for failure / success built-in
when used as a monad
m >>= g = \nothing just -> m nothing (\x -> g x nothing just)
VS
m >>= g = case m of
Nothing -> Nothing
Just x -> g x
The algebraic data type has to check that the result was indeed Just x
whereas the Church-encoding can jump right to the success continuation.
The problem is that your tree doesn't really make use of this advantage.
For that, you need a completely left-biased tree, like for example
Fork 1 (Fork 2 (Fork 3 ... Leaf) Leaf) Leaf
testTree = fromList [1..100000]
fromList = foldr (\x t -> Fork x t Leaf) Leaf
I've implemented this and posted it below your version at
http://hpaste.org/fastcgi/hpaste.fcgi/view?id=10686
I've used the criterion benchmarking package, so you can run this
straight out of the box.
Even then, the results are mixed. The Church-encoding shines in GHCi as
it should, but loses its advantage when the code is being compiled. I
guess we have to look at the core if we want to know what exactly is
going on.
PS: I'm not entirely sure I'm using criterion correctly, in particular
concerning strictness. For instance, dropping the fromJust will
reduce the run-time of the "data Maybe" benchmark by 75%. Comments?
Regards,
apfelmus
--
http://apfelmus.nfshost.com